Add R.has function and tests

I put together this PR to add the `R.has` function. It is similar to [underscore's](http://underscorejs.org/#has) `_.has` function, except with the arguments reversed to make partial application more useful. Here are a couple of examples of its use:
```javascript
var obj = {
  foo: 1,
  bar: 2,
};
R.has('foo', obj);  //=> true

var list = [{foo: 1}, {foo: 2}, {bar: 3}];
R.filter(R.has('foo'), list);  //=> [{foo: 1}, {foo: 2}]
```
This is one of the few functions that I find myself reaching back to underscore for so it would be great to get an even better version of this in Ramda!
@buzzdecafe I had thought 

    return obj[name] !== undefined;

but I think I do prefer

    return name in obj;

This is certainly what I'd want for a `has` function.  I could see the possibility of a `hasOwn` one as well, but I don't find any great need for it.